In certain applications, it is desirable to be able to quickly elevate the temperature of a liquid, particularly thermally sensitive liquids such as hydrocarbon fuels and lubricating oils. For instance, when automotive engines are operated in cold weather, it is highly desirable to be able to heat the fuel, particularly diesel fuel, above a certain threshold, called the cloud point, in order to raise its temperature to a level at which it may be readily pumped through the fuel filter to the engine when the engine is running. Diesel fuel below its cloud point normally will not pass through the tiny pores in the filter, and instead, because of its paraffin content, the fuel clogs the filter and causes the engine to stop.
In addition, electric fuel heaters, in the form of heat exchangers, can provide a warming of the fuel to facilitate starting of diesel engines in cold weather by pre-heating the fuel prior to cranking the engine, thus providing warmer, lower-viscosity, and hence more easily ignited fuel to the fuel injectors as starting of the engine commences.
Diesel engines, particularly truck engines and other heavy duty engines are notoriously hard to start when the ambient temperature is 32.degree. F. or lower. To address this problem, diesel fuel heaters, i.e., heat exchangers of various flat plate and tube-type designs, have been provided. Examples of such electric diesel fuel heaters are shown in U.S. Pat. Nos. 4,208,996; 4,349,001; and 4,477,715. Other examples of fuel heating assemblies include U.S. Pat. Nos. 4,372,279 and 4,091,265.
A problem arises in the design of heat exchangers for diesel heaters, however, in that the maximum physical size of such units is limited because of the limited space available in the engine compartment. This in turn imposes a constraint on the heater in the form of a limiting the amount of surface area available to make contact with the diesel fuel and in a limitation in terms of the maximum tolerable temperature of the heat exchanging surface itself. It is clear in the case of combustible liquids such as hydrocarbon fluids, e.g., diesel fuel, that the temperature of the heat exchange surface must be limited to a level below the flash point of the fluid for safe operation. For diesel fuel this upper limit is approximately 160.degree. F. In actuality, this limits the temperature differential of the heat exchanger surfaces or walls to a temperature value which is less than 160.degree.. At the other end of the scale, because the cloud point of diesel fuel can be as high as 32.degree. F., this means that the minimum temperature differential between the heat exchanger walls and the fuel being heated is limited to about 128.degree. F. Hence, energy must be transferred to the bulk of the fuel with a temperature differential of 128.degree. F. or less.
For conventional flat plate or tube type heat exchangers, the practical energy density is thus limited by the thermal resistance of the fuel to about 8 watts per square inch of surface area, a level which is impractical for the design of truly compact heat exchangers used as diesel fuel heaters.
The mathematical expression which defines the amount of heat which can be transferred by means of a heat exchanger is Q=UA.DELTA.T, where Q is the total energy transferred by the heat exchanger to the fluid to be heated; U is the overall heat-transfer coefficient; A is the physical dimensions of surface area in contact with the fluid; and .DELTA. T is the difference in temperature between the heat exchanger surface and the fluid. In analyzing this equation, it can be seen that the objective in the design of a heat exchanger is to optimize or maximize Q, the total energy transferred.
The typical methods for doing this are to (a) increase the area A by the addition of such mechanical elements such as fins, a plurality of conduits in contact with the fluid, etc., or (b) to attempt to increase the overall heat transfer coefficient U by promoting turbulent flow (turbulence) of the fluid to be heated through the heat exchanger. Typically this is accomplished by pumping the liquid past the heat exchanger surface at such a high velocity as to cause the liquid to undergo turbulent flow very early in the course of its passage through the heat-exchanger stage.
An expression for the effectiveness of a heat exchanger is to relate the heated exterior surface required to a predetermined unit of heat energy transferred, for example, 1 kilowatt at a fixed .DELTA. T, namely, 128.degree. F. Thus, a flat plate heat exchanger having an energy density of 8 watts/in..sup.2 capable of delivering 1 kw of energy to fuel at a .DELTA. T of 128.degree. F. would need 125 in..sup.2 of heated surface: 1000 watts/8 watts/ in..sup.2 =125 in..sup.2.
Heat exchangers which increase the internal (fluid side) surface temperature while maintaining the outer heated surface temperature constant have been demonstrated. These exchangers include extrusions of aluminum with internal fins. Such exchangers provide a practical increase of inner surface area by a factor of about 3, hence allowing the outer heated surface to run at 3 times the watt density, or about 24 watts/in..sup.2, resulting in heat exchangers requiring only 41.7 in..sup.2 of heated surface: 1000 watts/24 watts/in..sup.2 =41.7 in..sup.2.
In the first of the two solutions, there is a practical upper limitation on the physical size of the heat exchanger. Obviously the design of the engine, the engine compartment and the vehicle impose limitations on the space that is available for a heat exchanger, and hence there is an upper practical limit on the overall physical size of the heat exchanger and likewise the amount of surface area A.
With respect to the second solution, namely to increase the heat coefficient U, as has been indicated above, the closer the point of turbulent flow is brought relative to the entry point of the fluid and the more the overall design of the heat exchanger is directed to the concept of promoting and sustaining the condition of turbulent flow through the heat exchanger, the higher the heat exchange coefficient U becomes. This effect is limited however because of certain fluid dynamic characteristics which exist when a fluid contacts a surface such as the flat surface of a heat exchanger wall. The flow of a fluid, even a turbulent fluid, along a surface is characterized by the presence of a laminar layer or layers of fluid adjacent the surface of the heat exchanger. The fluid actually flows along the surface in the form of sheets or laminates of fluid of a finite thickness which are immediately adjacent that surface. These laminar layers represent the major thermal resistance in fluid systems.
The net effect of the presence of laminar boundary layers is to impair the efficiency of the heat exchanger. Laminar boundary layers are always present and cannot be eliminated, even at higher velocities. What occurs in moving the onset of turbulence closer to the inlet in the proper design of a heat exchanger is to shorten the length of the normal laminar flow zone adjacent the entry to the heat exchanger and to move the transition zone from laminar to turbulent closer to the leading edge or the entry point of the fluid to be heated. Even in the region of turbulent flow, it should be noted that a sublayer of laminar flow is still present. As is self-evident, one of the primary design criteria or objectives in the design of heat exchangers is to design a unit which is as small and as compact as it can be. The trade-off, however, when one endeavors to design a very compact or very short heat exchanger, is that the contribution or effect of the laminar boundary layer phenomenon is exaggerated because of the relatively small size and short length of the overall unit.
Another complication proceeding from small, compact heat exchangers is that if the heat source in the heat exchanger is to be a simple, planar electric heating element, then the surface area component A of the overall heat exchange equation is also limited to approximately the area of the planar heating element employed. While extended surfaces such as internal fins can improve the area factor somewhat, it is known that the efficiency of fins in liquids is low, resulting in higher weight per unit of heat transferred.
A further goal for heat exchangers, especially for diesel engines, is to minimize the pressure drop or "head loss" across the heat exchanger, while maximizing the overall heat transfer coefficient. This is especially important for diesel fuel systems because a limited amount of pumping force is available, i.e., 14 psi (one atmosphere of vacuum) on the fuel tank side of the fuel filter because the primary fuel pump is on the engine side of the fuel filter. Normally, a maximum pressure of 10 inches of mercury is allowed for the combination of fuel line, heater, filter and fittings with most of this being accounted for by the fuel filter element. Thus, it can be seen that, with conventional currently available heat exchangers, generating turbulent flow in a diesel fuel heater by causing a restricted flow path and hence high velocity results in high pressure drop or "head loss," an unacceptable method for increasing the overall heat transfer coefficient sufficiently to construct compact, high power diesel fuel heaters. The present invention provides a solution to the problems which have just been discussed.